This invention relates to a Coriolis mass flow/density sensor with a single straight measuring tube.
U.S. Pat. No. 5,531,126 describes a Coriolis massflow/density sensor which can be installed in a pipe by means of a connecting element at the inlet end and a connecting element at the outlet end and through which a fluid to be measured flows during operation, comprising:
a single straight measuring tube having a longitudinal axis and extending between and fixed to the connecting elements;
a straight dummy tube extending parallel to the measuring tube and not traversed by the fluid;
a nodal plate on an inlet side and a nodal plate on an outlet side,
one of which fixes the inlet-end portion of the measuring tube to the corresponding end portion of the dummy tube, and
the other of which fixes the outlet-end portion of the measuring tube to the corresponding end portion of the dummy tube, so that the measuring tube and the dummy tube are arranged side by side;
a support tube having its ends fixed to the respective connecting elements and having a longitudinal axis of symmetry parallel to the longitudinal axis of the measuring tube; and
means which act only on the dummy tube to excite the measuring tube into flexural vibrations whose frequency is not, however, identical with the resonance frequency of the measuring tube, with the measuring tube and the dummy tube vibrating in antiphase.
This prior-art Coriolis mass flow/density sensor is mechanically balanced only in a narrow range of density valuesxe2x80x94approximatelyxc2x1 5% of a rated densityxe2x80x94for a given dimensional design, i.e., only at these density values will forces originating from the vibrations of the measuring tube be practically not transmitted via the connecting elements to the pipe. The above range is extended by the excitation xe2x80x9cbesidexe2x80x9d the resonance frequency, but substantially more excitation energy is required than for excitation at the resonance frequency. The less balanced the mass flow/density sensor is, the more such forces and vibrational energy will be transmitted to the pipe; thus, however, vibrational energy is lost and measuring inaccuracy increases.
This unbalance has a disturbing effect not only in case of temperature-induced changes in the density of one and the same fluid but also particularly during the measurement of different fluids flowing in the pipe at different times, for example one after another.
Since Coriolis mass flow/density meters should be suitable for measuring as wide a range of very different fluids with different densities as possible, it is therefore important to provide Coriolis mass flow/density sensors which are balanced in the above sense over a wide density range and thus measure accurately.
To accomplish this, a first variant of the invention provides a Coriolis mass flow/density sensor which can be installed in a pipe and through which a fluid to be measured flows during operation, comprising:
a single straight measuring tube having a longitudinal axis, an inlet end, and an outlet end;
a support fixed to the inlet end and the outlet end,
a longitudinal centroidal line of which is parallel to, but does not coincide with, the longitudinal axis of the measuring tube;
a cantilever
which is fixed to the measuring tube midway between the inlet end and the outlet end, and
which during operation causes the measuring tube to vibrate either in a first fundamental flexural mode or in a second fundamental flexural mode having a higher frequency than the first fundamental flexural mode;
an excitation arrangement for constantly exciting the measuring tube in the second fundamental flexural mode
which is disposed approximately midway between the inlet end and the outlet end; and
a sensor for the motions of the measuring tube on an inlet side and a sensor for the motions of the measuring tube on an outlet side which are located between the middle of the measuring tube and the inlet end and outlet end, respectively, at the same distance therefrom.
In a first preferred embodiment of the first variant of the invention, the support is a cylindrical tube having a wall of uniform thickness and a longitudinal axis which is parallel to, but does not coincide with, the longitudinal axis of the measuring tube.
In a second preferred embodiment of the first variant of the invention, the support is a cylindrical tube having a wall of only partially uniform thickness and a longitudinal axis which is parallel to, or coincides with, the longitudinal axis of the measuring tube, with the tube wall in the region of a first generating line diametrically opposite the cantilever being at least partially thicker than the uniform wall thickness and/or the tube wall in the region of a first generating line adjacent to the cantilever being at least partially thinner than the uniform wall thickness in order to form a counterbalance.
According to a development of the second embodiment of the first variant of the invention, a counterweight is attached, partially inserted in, or integrally formed on the tube wall diametrically opposite the cantilever.
In a third preferred embodiment of the first variant of the invention, which can be used in the above embodiments and the development of the second embodiment, the cantilever has the form of a plate or disk having a bore by means of which the plate or disk is slipped over the measuring tube. The plate or disk preferably consists of a semicircular ring portion and a rectangular portion formed thereon, the semicircular ring portion being coaxial with the bore. Advantageously, the plate or disk has a thickness equal to approximately half the diameter of the measuring tube.
According to a development of the first variant of the invention and its embodiments, the measuring tube is provided with an annular rib on the inlet side and an annular rib on the outlet side which are disposed at the locations of the respective sensors.
In a fourth preferred embodiment of the first variant of the invention, the excitation arrangement consists of
a first portion which acts on the measuring tube in the direction of the intersection of a longitudinal axis of symmetry of the cantilever and the longitudinal axis of the measuring tube with a first excitation force, and a
second portion, which acts on an end of the cantilever remote from the measuring tube with a second excitation force directed opposite to the first excitation force.
A second variant of the invention provides a Coriolis mass flow/density sensor which can be installed in a pipe and through which a fluid to be measured flows during operation, comprising:
a single straight measuring tube having an inlet end and an outlet end;
an inlet plate fixed at the inlet end and surrounding the measuring tube;
and outlet plate fixed at the outlet end and surrounding the measuring tube;
a first support plate fixed to the inlet plate and the outlet plate and extending parallel to a first generating line of the measuring tube;
a second support plate fixed to the inlet plate and the outlet plate and extending parallel to a second generating line of the measuring tube diametrically opposite the first generating line;
a cantilever
which is fixed to the measuring tube midway between the inlet end and the outlet end, and
which during operation causes the measuring tube to vibrate either in a first fundamental flexural mode or in a second fundamental flexural mode having a higher frequency than the first fundamental flexural mode;
a longitudinal bar located opposite the cantilever and fixed to the first and second support plates, said longitudinal bar acting as a counterbalance;
an excitation arrangement
which constantly excites the measuring tube in the second fundamental flexural mode, and
which is disposed approximately midway between the inlet end and the outlet end; and
a sensor for the motions of the measuring tube on an inlet side and a sensor for the motions of the measuring tube on an outlet side which are located between the middle of the measuring tube and the inlet end and outlet end, respectively, at the same distance therefrom.
In a first preferred embodiment of the second variant of the invention, the cantilever is a plate having a front surface, a back surface, an axis of torsional vibration parallel to the axis of the measuring tube, and a bore by means of which the plate is slipped over the measuring tube, said plate consisting of a circular-segment portion, which is coaxial with the bore, and a rectangular portion formed thereon, an end surface of which, which is cut centrally by a diameter of the measuring tube, is fixed to a fastening area of a beam which is longer than the end surface and has a first end and a second end which project beyond the end surface and comprise respective continuations of the fastening area.
According to a development of this first embodiment of the second variant of the invention, the excitation arrangement consists of a first excitation system, fixed to the continuation of the fastening area of the first beam end, and a second excitation system, fixed to the continuation of the fastening area of the second beam end, with the first and second excitation systems comprising a first coil and a second coil, respectively, which in operation are traversed by an exciting current in opposite directions.
According to a second development of the second variant of the invention, which can also be used in the first preferred embodiment of the second variant, in order to suppress modes of vibration other than the second fundamental flexural mode,
a first part of a first brake assembly based on the eddy-current principle is fixed to the front surface of the plate in an area in which the axis point through said plate;
a first part of a second brake assembly based on the eddy-current principle is fixed to the back surface of the plate in an area in which the axis of torsional vibration has a possible piercing point through said plate;
the first brake assembly comprises a second part which is attached to a first holder fixed at least to the first support plate; and
the second brake assembly comprises a second portion which is attached to a second holder fixed at least to the first support plate.
In a preferred embodiment of the first development of the second variant of the invention, the first parts of the brake assemblies are circular cylindrical permanent magnets, and the second parts of the brake assemblies are copper disks.
The two variants of the invention and their embodiments and developments may be further improved by extending the measuring tube beyond the inlet and outlet ends using respective tube sections of equal length whose respective free ends are fixed in a housing.
According to a further development of the invention, in addition to the second fundamental flexural mode, the first fundamental flexural mode is excited.
One advantage of the invention is that the accuracy of the mass flow measurement is excellent over a wide density range (0 kg/m3 to 3000 kg/m3; 0 kg/m3 corresponds to a null measurement with a vacuum in the measuring tube). Mass flow/density sensors of a preproduction series, for example, showed accuracies within 0.1% of the measured value.
Another important advantage of the invention is that it is also well suited for measuring the viscosity of the fluid, which is based on the following facts, which are familiar to those skilled in the art:
The viscosity of a fluid can be measured with a Coriolis mass flow/density sensor only if the measuring tube or tubes of the sensor (also) perform a torsional vibration, so that shear forces are exerted on the fluid. In the case of straight measuring tubes excited in flexural modes of vibration, no torsional vibrations, and thus no shear forces, occur at all.
In the case of bent, particularly U-shaped, measuring tubes, torsional vibrations do occur, but their amplitude is so small that a viscosity measurement is virtually impossible. Patent documents in which viscosity is mentioned in connection with Coriolis mass flow/density sensors are not very numerous.
U.S. Pat. No. 4,938,075, for example, only mentions the Navier-Stokes equation, which includes the shear viscosity, which is not measured, however. Other patent specifications deal only with the viscosity compensation of the measured mass-flow values; see, for example, U.S. Pat. Nos. 5,027,662, 4,876,879, and 4,872,351.
Single-tube Coriolis mass flow/density sensors according to the invention perform not only the flexural vibration required and desired for mass-flow and density measurements but also, because of the cantilever, a torsional vibration in the second fundamental flexural mode around an axis whose position is explained below.
In the invention, the amplitude of this torsional vibration is sufficient to permit the viscosity of the fluid to be measured in addition to mass flow and density with only little additional electronic circuitry.
For this viscosity measurement, recourse can be had to methods described in the literature which discuss this measurement in connection with the measurement of fluid density using vibrating mechanical arrangements, particularly tubes.
According to the journal xe2x80x9cIEEE Transactions on Industrial Electronics and Control Instrumentationxe2x80x9d, August 1980, pages 247 to 253, for example, viscosity can be determined if the resonance quality factor of the vibrating mechanical arrangement, including the fluid, is measured. This can be done, for example, by measuring the electric current with which the arrangement is excited.